Brian,
The XL175 carb may work, although the jetting looks really lean for the MR so you'd probably have to change that. If you already have it, you might as well try it.
The stock 28mm MR175 carb was used on several models (MT250, FL250, early CR125M) and shows up all the time on eBay. I've gotten a few of them there for about $20 to $30 shipped.
I believe the CR125M and MT125 tanks may bolt right on (I'm not 100% sure), but when I test-fitted my CR tank I found it was much narrower than the MR seat, and looked strange. The Clarke 11362 plastic tank l use on my bike looks great and holds a lot of gas, but it isn't cheap (I paid $209 for one, plus $150 for a spare one I'll never use). It requires drilling a hole through the frame gussets to mount properly.
Here's a pic:
http://i399.photobucket.com/albums/pp74 ... 010001.jpgRay